1. Field of the Invention
The present invention relates to a method of returning a refrigerator oil of an air conditioner by which in operating a refrigerating cycle constituted to circulate a mixture of an HFC refrigerant and refrigerator oil by a compressor, when the refrigerator oil discharged from the compressor along with the refrigerant is adhered to inner walls or the like of refrigerant pipes, a heat side heat exchanger, user side heat exchangers and the like in the refrigerating cycle and the refrigerator oil present in the compressor is reduced, a recovery operation carried out for returning the refrigerator oil can be performed in a short period of time and reliably.
2. Description of Related Art
Generally, in an air conditioner, an outdoor unit mounted with a compressor, an accumulator, heat side heat exchangers and the like and room units mounted with user side heat exchangers, expansion valves and the like, are connected by refrigerant pipes to establish a refrigerating cycle, and a mixture of a refrigerant and a refrigerator oil is circulated in the refrigerating cycle.
The mixture is made to flow from the outdoor unit to the room units, subjected to heat exchange (condensing operation or evaporating operation) at the user side heat exchangers, returned to the accumulator, and thereafter drawn again to the compressor. However, depending on the difference of elevation between the outdoor unit and the room units, a prolonged pipeline and the solubility of the refrigerant in respect of the refrigerator oil, some of the refrigerator oil may not be returned to the accumulator, but adheres to or remains at the inner walls of the refrigerant pipes, the inside of the user side heat exchangers and the like.
Hence, there are many air conditioners having an outdoor unit installed on the roof, to evaluate return of the refrigerator oil at predetermined intervals in order to avoid the amount of the refrigerator oil in the compressor from decreasing to less than a necessary amount.
The operation of returning the refrigerator oil is carried out by, for example, fully opening expansion valves of the room units operating as pressure reducing devices (or the opening degree is enlarged more than that in normal heating and cooling operation in accordance with the capacity of the room units) and by operating the compressor at maximum capacity.
That is, the amount of supply of the mixture of the refrigerant and the refrigerator oil to the room units is increased by operating the compressor at high power by which the flow rate of the mixture is accelerated in the refrigerant pipes and the refrigerator oil is returned by blowing off the refrigerator oil adhered to the inner walls of the refrigerant pipes and the like.
Meanwhile, the solubility of the refrigerant in respect of the refrigerator oil differs depending on the kind of the refrigerant, and when the solubility of the refrigerant is low (when the compatibility is poor), the viscosity of the refrigerator oil is increased as a result. For example, in the case of an HFC refrigerant, the solubility of the refrigerant in respect of the refrigerator oil is lower than the solubilities of CFC and HCFC refrigerants in respect of the refrigerator oil, and when the same refrigerator oil is used the viscosity of the refrigerator oil in refrigerant pipes is increased more than those of the CFC and HCFC refrigerants. Therefore, a larger flow rate of the refrigerant is required to return the refrigerator oil adhered to the refrigerant pipes.
Meanwhile, in the case of a so-called multi-unit type air conditioner where a plurality of room units are connected to one outdoor unit, the refrigerant pipes are long and complicated. Further, the amount of the refrigerator oil is determined based on the maximum capacity of a compressor, and accordingly the refrigerator oil is generally decreased in comparison with the amount of the refrigerant, and it becomes important to return effectively the refrigerator oil by a recovery operation.
Normally, in the case of such a multi-unit type air conditioner as described above, a total of the capacities of the plurality of room units is actually set to be larger than the capacity of the compressor of the outdoor unit. This is mainly due to the following reason.
The load of air conditioning in rooms is varied during one day and for example, considering the refrigerating operation, in the morning the load of an east side room is increased by direct sunlight, and in the afternoon, the load of a west side room is increased. Accordingly, by selecting the capacity of the outdoor unit in accordance with a total of loads in all the rooms, the investment cost and a space for installing the outdoor unit can be reduced.
Therefore, in the recovery operation for returning the refrigerator oil adhered to the refrigerant pipes for connecting the outdoor unit and the room units or devices in the room units to the outdoor unit (compressor), even when, for example, expansion valves of all the room units are fully opened and the compressor is driven by the maximum capacity, if an HFC refrigerant is used, an amount of circulating the refrigerant necessary for rated capacities of the respective room units cannot be provided. That is, the necessary flow rate of the refrigerant cannot be provided, and therefore complete recovery (returning) of the refrigerator oil cannot be achieved.